The present invention concerns a process for coating of substrates by means of plasma polymerisation. The invention also concerns a coating, produced using the process, of a polymer substrate and applications of the process.
Polymer substrates such as in particular flexible substrates are coated amongst other reasons in order to influence the surface composition or appearance of the polymer or protect the surface mechanically, physically and chemically. This may be to increase the adhesion to the surface or the printability, to prepare the surface for further functional coatings, to ensure protection against abrasion or damage, to reduce or prevent the permeability of certain gases or liquids on or through the surface of the substrate, or to increase the chemical resistance of the substrate to certain chemicals.
For surface treatment of polymer substrates which increases the polarity or surface tension in the short term, a multiplicity of methods are known where in principle two processes occur most commonly: modification of the surface for example by a corona discharge at atmospheric pressure or by a plasma process at reduced pressure.
Both said processes are important in particular in connection with the increase in adhesion to the polymer substrate or the increase in printability. However, in corona discharge it has been found that the printability for example of polymer packing films is good only immediately after performance of the treatment and the printability diminishes again after just a few hours or days.
In contrast, in a series of documents it is proposed to modify or coat the polymer by means of a low pressure plasma process, where the coating is usually hydrophilic and allows good adhesion or printability. This printability is retained practically without restriction because of the coating.
Thus for example in JP-59-15569 and WO, A1AU89/00220 it is proposed to coat a polymer substrate by means of plasma polymerisation of an organic compound, together for example with a working gas and water or water vapour. It is also proposed in WO95/04609 to treat or coat the surface by means of plasma polymerisation of an organic compound in the presence of hydrogen peroxide.
U.S. Pat. No. 3,397,132 concerns a coating of metal surfaces, where an electric discharge occurs in the presence of organic gases and an inert carrier gas. With regard to the inorganic gases, the absence of water is neither mentioned nor otherwise stated as essential. In contrast, precise statements are made for other parameters such as pressure, temperature, concentration, voltage and frequency. Corresponding modifications to the parameters achieve the desired improvements in the metal surfaces by plasma coating.
In a polar plasma coating to DE, A1 3908418, at least one organic compound and an optional inorganic gas is used. Plastic containers are coated on the inside with coatings impermeable to organic solvents, where the inside of the container is impacted with a low pressure plasma. This process too does not mention the absence of water.
Firstly, the coatings proposed in the state of the art have a poor adhesion to the substrate, or they have restricted wettability. The use of peroxide or water and oxygen causes a problem as the resulting xe2x80x9cworking gasxe2x80x9d is aggressive and can attack the surface of the substrate (etching).